refactor: use *SPI everywhere to make consistant for implementations.

Fixes #4663 "in reverse" by making SPI a pointer everywhere, as discussed
in the comments.

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/machine/board_maixbit_baremetal.go

@@ -6,10 +6,10 @@ import "device/kendryte"
 
 // SPI on the MAix Bit.
 var (
-	SPI0 = SPI{
+	SPI0 = &SPI{
 		Bus: kendryte.SPI0,
 	}
-	SPI1 = SPI{
+	SPI1 = &SPI{
 		Bus: kendryte.SPI1,
 	}
 )